From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-13.3 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI, MENTIONS_GIT_HOSTING,SIGNED_OFF_BY,SPF_PASS,URIBL_BLOCKED,USER_AGENT_MUTT autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 1C41AC43381 for ; Thu, 14 Mar 2019 03:15:41 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id D3D0120854 for ; Thu, 14 Mar 2019 03:15:40 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="I+H3yalP" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726418AbfCNDPg (ORCPT ); Wed, 13 Mar 2019 23:15:36 -0400 Received: from mail-ed1-f67.google.com ([209.85.208.67]:39763 "EHLO mail-ed1-f67.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726141AbfCNDPg (ORCPT ); Wed, 13 Mar 2019 23:15:36 -0400 Received: by mail-ed1-f67.google.com with SMTP id p27so3331248edc.6; Wed, 13 Mar 2019 20:15:34 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to:user-agent; bh=w6tTH8LHrpkeFIvESsQeWU08KFpl98E7IcxYbXr9Pzs=; b=I+H3yalPQvE1XM5roeTlEz235MZ3rHeexoAorISDeN6dTLGINpb3r4Hdmi7r3BdKLw kWHthXDXZgNBqjt7VU4+ie+DdWcdw82rCbG2UtdtOQlMzq4ChuY/ZuYTZ4zpMS6Hec/6 z2KVuFPpDPrenocZxjDf+frHUz3prWXeocSzXWoDG8HVvG8AX6CK7evxVx8DPmbhFD6m dXzFWGIlroHUugwy4rWXArV4EW1aCtKEKhzf1JpTDRgLM05iZPGK4e7Osf7UVjCQV1o6 o19r50zzePtV8o8t5AWhYorJa64vcnXzZS+jEfhfgIxkgZhCGCA3lirCBn+1nXEQLPLv r6YQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to:user-agent; bh=w6tTH8LHrpkeFIvESsQeWU08KFpl98E7IcxYbXr9Pzs=; b=ZJbbTwk/xWzallFSg9LoDuucG+HKHw9I/qET712Oe99JSW9mc6W59PNSr4VC80q5Q1 MUKUB60jNfWiEbX2kgrIPa48LbdA5Hpv9k9VVXhZpkgQJOlDz6Nj5hE2xF8YvpIT1rcv x+rzdE/i3vxBh+JBPn9hyRf1NaWXMJWMCO7MfV6KiwIsgY/oUe1uB//PNA5jfK7r10pm 3c/ZDtDJmbmrNn9GaLyPNsCiYCvEwu3DMZ+21UH80oR7MYKNS6k0PN1HE566REsCAPs7 XsCbbrRc44eHgm9bZNWwj0iaE0WWJ1KegIAGjAY29o0OkSMJhCeRwmAst7KQxmqnxNPJ HdzA== X-Gm-Message-State: APjAAAWCRNYekOrAi2j5lTvECX0Ntoxxkg+dxD9a99XHfF3ZCxKNklLt ASV5aM4YcP41m+z1I2wpFrs= X-Google-Smtp-Source: APXvYqwz5szNib+ok+DvL6/VQ+HRwf4y2kR0kamI3yar6UIcVR5BOSrrnk1r5UHnD0ICz45QEhZcQg== X-Received: by 2002:aa7:cfc7:: with SMTP id r7mr9484252edy.46.1552533333582; Wed, 13 Mar 2019 20:15:33 -0700 (PDT) Received: from archlinux-ryzen ([2a01:4f9:2a:1fae::2]) by smtp.gmail.com with ESMTPSA id 41sm1099284edu.85.2019.03.13.20.15.31 (version=TLS1_3 cipher=AEAD-AES256-GCM-SHA384 bits=256/256); Wed, 13 Mar 2019 20:15:32 -0700 (PDT) Date: Wed, 13 Mar 2019 20:15:30 -0700 From: Nathan Chancellor To: Nick Desaulniers Cc: akpm@linux-foundation.org, clang-built-linux@googlegroups.com, linux-kbuild@vger.kernel.org, stable@vger.kernel.org, Adhemerval Zanella , Arnd Bergmann , James Y Knight , Masahiro Yamada , Rasmus Villemoes , Steven Rostedt , Namhyung Kim , Greg Kroah-Hartman , Alexander Shishkin , Dan Williams , Andy Shevchenko , linux-kernel@vger.kernel.org Subject: Re: [PATCH v4] lib/string.c: implement a basic bcmp Message-ID: <20190314031530.GA9264@archlinux-ryzen> References: <7549EE7E-4172-467D-815A-63664A33D410@goodmis.org> <20190313211335.165605-1-ndesaulniers@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190313211335.165605-1-ndesaulniers@google.com> User-Agent: Mutt/1.11.4 (2019-03-13) Sender: stable-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: stable@vger.kernel.org On Wed, Mar 13, 2019 at 02:13:31PM -0700, Nick Desaulniers wrote: > A recent optimization in Clang (r355672) lowers comparisons of the > return value of memcmp against zero to comparisons of the return value > of bcmp against zero. This helps some platforms that implement bcmp > more efficiently than memcmp. glibc simply aliases bcmp to memcmp, but > an optimized implementation is in the works. > > This results in linkage failures for all targets with Clang due to the > undefined symbol. For now, just implement bcmp as a tailcail to memcmp > to unbreak the build. This routine can be further optimized in the > future. > > Other ideas discussed: > * A weak alias was discussed, but breaks for architectures that define > their own implementations of memcmp since aliases to declarations are > not permitted (only definitions). Arch-specific memcmp implementations > typically declare memcmp in C headers, but implement them in assembly. > * -ffreestanding also is used sporadically throughout the kernel. > * -fno-builtin-bcmp doesn't work when doing LTO. > > Link: https://bugs.llvm.org/show_bug.cgi?id=41035 > Link: https://code.woboq.org/userspace/glibc/string/memcmp.c.html#bcmp > Link: https://github.com/llvm/llvm-project/commit/8e16d73346f8091461319a7dfc4ddd18eedcff13 > Link: https://github.com/ClangBuiltLinux/linux/issues/416 > Cc: stable@vger.kernel.org > Reported-by: Nathan Chancellor > Reported-by: Adhemerval Zanella > Suggested-by: Arnd Bergmann > Suggested-by: James Y Knight > Suggested-by: Masahiro Yamada > Suggested-by: Nathan Chancellor > Suggested-by: Rasmus Villemoes > Signed-off-by: Nick Desaulniers > Acked-by: Steven Rostedt (VMware) Reviewed-by: Nathan Chancellor Tested-by: Nathan Chancellor Thanks for fixing this better than I did (or tried to)! > --- > Changes V3 -> V4: > * Include the entirety of Rasmus' sugguestion, as per Steven. > * Change formal parameter identifiers to match the comment. > Changes V2 -> V3: > * Adjust comment as per Steven to Rasmus' sugguestion. > * Pick up Steven's Ack. > Changes V1 -> V2: > * Add declaration to include/linux/string.h. > * Reword comment above bcmp. > > include/linux/string.h | 3 +++ > lib/string.c | 20 ++++++++++++++++++++ > 2 files changed, 23 insertions(+) > > diff --git a/include/linux/string.h b/include/linux/string.h > index 7927b875f80c..6ab0a6fa512e 100644 > --- a/include/linux/string.h > +++ b/include/linux/string.h > @@ -150,6 +150,9 @@ extern void * memscan(void *,int,__kernel_size_t); > #ifndef __HAVE_ARCH_MEMCMP > extern int memcmp(const void *,const void *,__kernel_size_t); > #endif > +#ifndef __HAVE_ARCH_BCMP > +extern int bcmp(const void *,const void *,__kernel_size_t); > +#endif > #ifndef __HAVE_ARCH_MEMCHR > extern void * memchr(const void *,int,__kernel_size_t); > #endif > diff --git a/lib/string.c b/lib/string.c > index 38e4ca08e757..3ab861c1a857 100644 > --- a/lib/string.c > +++ b/lib/string.c > @@ -866,6 +866,26 @@ __visible int memcmp(const void *cs, const void *ct, size_t count) > EXPORT_SYMBOL(memcmp); > #endif > > +#ifndef __HAVE_ARCH_BCMP > +/** > + * bcmp - returns 0 if and only if the buffers have identical contents. > + * @a: pointer to first buffer. > + * @b: pointer to second buffer. > + * @len: size of buffers. > + * > + * The sign or magnitude of a non-zero return value has no particular > + * meaning, and architectures may implement their own more efficient bcmp(). So > + * while this particular implementation is a simple (tail) call to memcmp, do > + * not rely on anything but whether the return value is zero or non-zero. > + */ > +#undef bcmp > +int bcmp(const void *a, const void *b, size_t len) > +{ > + return memcmp(a, b, len); > +} > +EXPORT_SYMBOL(bcmp); > +#endif > + > #ifndef __HAVE_ARCH_MEMSCAN > /** > * memscan - Find a character in an area of memory. > -- > 2.21.0.360.g471c308f928-goog >